1. Field of the Invention
The present invention concerns a process for preparation by the wet method of insulating refractory products having controlled porosity.
2. Description of the Prior Art
It is known that the manufacturing of devices for casting, purifying and handling molten metals and more particularly corrosive molten metals such as aluminium, is conditioned by the perfectioning of refractory materials capable simultaneously of bearing the high temperature at which these molten metals are kept, of withstanding effectively their corrosive and erosive action and of bearing the thermal shock to which the walls of these devices are subjected.
During the last few years, various efforts have been made with a view to perfecting such materials. In general, these efforts have made it possible to obtain refractory products of the ceramic type which withstand well the aggressiveness of metals. But these products are not very resistant to thermal shocks.
It was disclosed by W. D. Kingery, Introduction to Ceramics, 1960, that increasing the porosity of a given refractory material lends to better spalling resistances up to an optimum porosity of 10 to 20%. However, a high porosity appeared to mean a wetting of the pores of the refractory by the molten metal, and therefore a poor resistance to erosion, so that the refractory products proposed up to now for containers or pumps for molten metals of a relatively low melting point such as molten aluminium and aluminium-base alloys, molten zinc and molten zinc-base alloys, were of a relatively low porosity and therefore of a relatively low resistance to thermal shock, and had to be frequently replaced in use.
The wet method of preparation of refractories consists in mixing in the dry state the components of the refractory, in dispersing them in water with a foaming agent to form a foamed slip, in stabilizing the foam by the addition of a stabilizing agent, in pouring the foamed slip into lubricated metal moulds, then in drying the articles and firing them.
That method was not used up to now for the manufacture of products of a very high porosity, for instance higher than 50%, since such products were believed to have too little cohesion to withstand erosion by molten metals. In a known method, the product obtained after drying was fired at a temperature in the order of 1.700.degree. C., and therefore sintered, so that there remained little porosity. In another known method disclosed in U.S. Pat. Nos. 3,294,562 and 3,269,849 of Caprio et al., the firing was effected at a relatively low temperature, but the slip was pressed into a slab or board-like form under high pressure before being dried and fired, so that its porosity was also low.
It is an object of the present invention to provide a process of preparation by the wet method of refractory products of a high porosity showing high resistances to thermal shock and to erosion and corrosion by molten metals such as molten aluminium and aluminium-base alloys.
It is another object of the invention to provide a process of preparation by the wet method of refractory products of a coefficient of heat conduction between 0.1 and 0.5 Kcal/m/sq.m/d.degree.C./hour, and therefore of an excellent thermal insulation.
It is another object of the invention to provide a process of preparation by the wet method of refractory products very stable both chemically and dimensionally.
It is still another object of the invention to provide a process of preparation by the wet method of containers, pumps or like articles for the handling of molten metal such as molten aluminium and aluminium-base alloys, of a long service life.